This invention pertains to systems and methods for the detection of cardiac arrhythmias and, in particular, for the detection of premature atrial contractions.
Two broad classes of cardiac rhythm management devices are pacemakers and implantable cardioverter/defibrillators. Cardiac pacemakers are medical devices, usually implantable, that provide electrical stimulation in the form of pacing pulses to selected chambers of the heart (i.e., the atrium and/or ventricle) in order to either make up for the heart""s inability to pace itself at an appropriate rhythm in order to meet metabolic demand (bradycardia pacing) or to therapeutically treat abnormal rhythms such as tachycardias (antitachycardia pacing). Pacemakers typically have a programmable electronic controller that causes the pacing pulses to be output in response to lapsed time intervals and sensed electrical activity (i.e., intrinsic heart beats not due to a pacing pulse). Implantable pacemakers sense intrinsic cardiac electrical activity by means of internal electrodes disposed in or near the chamber to be sensed. A depolarization wave associated with an intrinsic contraction of the atria or ventricles that is detected by the pacemaker is referred to as an atrial sense (A-sense) or ventricular sense (V-sense), respectively. In order to cause a contraction in the absence of an intrinsic beat, a pacing pulse (either an atrial pace or a ventricular pace) with energy above a certain pacing threshold must be delivered to the chamber. Cardioversion and defibrillation refer to electrical shocks delivered to the heart in order to terminate a tachycardia, including atrial or ventricular fibrillation. The electric shock terminates the tachycardia by depolarizing all excitable myocardium which thereby prolongs refractoriness, interrupts reentrant circuits, and discharges excitatory foci. Implantable cardioverter/defibrillators (ICD""s) provide this kind of therapy by delivering a shock pulse to the heart when fibrillation or other type of tachycardia is detected by the device.
Premature atrial contractions (PACs) are atrial contractions resulting from ectopic excitatory foci in the atria that occur prior to the time they would normally be expected. Such premature excitation of the atria spreads to the atrio-ventricular (AV) node and may or may not be conducted to the ventricles depending upon whether the AV node is still refractory from the preceding beat. Ectopic foci can be due to local areas of ischemia, but premature atrial contractions can also occur in normal persons when provoked by drugs, alcohol, caffeine, or tobacco. Premature atrial contractions may depress cardiac output (due to lessened diastolic filling of the ventricles) and can also precipitate other tachyarrhythmias.
One way of detecting PACs is to measure the interval between A-senses, referred to as the Axe2x80x94A interval, and declare an A-sense to be a PAC if the current Axe2x80x94A interval is less than a specified percentage of the preceding Axe2x80x94A interval. The problem with methods that base a PAC detection decision upon a single preceding Axe2x80x94A interval, however, is a lack of specificity. That is, such methods can be fooled into defining events as PACs that are not PACs. For example, some individuals exhibit occasional sinus pauses when exercising. A PAC detection method based upon a single preceding Axe2x80x94A interval will call every event after those pauses a PAC even though they are physiologic and normal. It is toward the objective of reducing such false positive results that the present invention is primarily directed.
The present invention is an improved system and method for detecting PACs based upon measurement of preceding Axe2x80x94A intervals. Atrial events corresponding to depolarizations or paces are detected with an atrial sensing channel, and corresponding Axe2x80x94A intervals are measured. A present Axe2x80x94A interval, defined as the time interval between a presently detected atrial event and a previously detected atrial event, is measured, and a moving average of such Axe2x80x94A intervals is computed. The Axe2x80x94A interval moving average is preferably computed from at least four Axe2x80x94A intervals. In accordance with the invention, a detected atrial depolarization is classified as a premature atrial contraction if the present Axe2x80x94A interval is less than a specified value based upon the Axe2x80x94A interval moving average, such as a specified fraction of the moving average. In a preferred embodiment, the fraction of the Axe2x80x94A interval moving average that the present Axe2x80x94A interval must be less than in order for a premature atrial contraction to be detected is set to seventy-five percent. To obtain further specificity, the method may limit the value of an Axe2x80x94A interval used to compute the moving average Axe2x80x94A interval to being no longer than a specified maximum value, such as approximately 2000 milliseconds. An additional detection criteria may be employed in which the present Axe2x80x94A interval is required to also be less than a specified absolute value in order for a premature atrial contraction to be detected. In an exemplary embodiment, the specified absolute value is set to approximately 600-750 milliseconds.